On Sat, 1 Aug 2020 09:19:40 +0100, Tim Lamb wrote:

You mean the sheets that have been cut I to pieces to fit snugly
*between* joists?

Hand held or bench circular saw, then finish the cut with a hand saw.

Power tool to cut celotex type rigid foam board? Seems a bit OTT.

A hand saw will do it but generate a lot of bits that'll be great fun
with any static electricity (but probably not as fun as the bits from
expanded polystyrene). Deep score with a stanley knife 'n snap works
for 25 mm not tried that on thicker boards. For thicker borads I'd
try a scallop bladed knife, possibly with some thin wedges to keep
the cut open.

--
Cheers
Dave.